Download BAM EARTHQUAKE, IRAN: LESSONS ON THE SEISMIC

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
BAM EARTHQUAKE, IRAN: LESSONS ON THE SEISMIC BEHAVIOUR OF
BUILDING STRUCTURES
A. R. Manafpour
1
1
Structural Engineer, Halcrow Group Limited , Power and Structures, Glasgow, UK
ABSTRACT :
The Bam earthquake on 26 December 2003 with magnitude Mw=6.6 destroyed most of the city of Bam in Iran and
nearby villages, and killed more than 26,000 people. The earthquake was by far the most devastating earthquake in
the history of the region around Bam. After the earthquake the author undertook a field investigation and visited the
affected area. The paper studies the structural damage on traditional as well as modern building structures during the
earthquake. Adobe, masonry, steel and reinforced concrete structures are considered with some examples
demonstrating the response of each type of the buildings. Discussion of the structural behavior is given with reference
to existing seismic design codes and construction practice within the region. It is concluded that as a considerable
number of buildings in central and eastern provinces of Iran, more specifically in villages, are built of mud-bricks and
will not resist similar magnitude earthquakes. Due to specific materials and construction forms special strengthening
procedures needs to be developed.
KEYWORDS:
Bam Earthquake, Adobe Structures, Earthquake Effects, Earthquake Filed Investigation, Seismic
1. INTRODUCTION
A large earthquake with a magnitude of Mw=6.6 (USGS[3]) struck the city of Bam, located approximately
1000km southeast of Tehran, at 05:26:56 local time (01:56:56 GMT) on Friday 26th December 2003. The
earthquake destroyed most of Bam city and the nearby villages and the official death toll exceeded 26,000 with
more than 30,000 injuries and 75,000 left homeless.
The earthquake was strongly felt in the provincial capital of Kerman, about 190km (120 miles) Northwest of
Bam, however the main damage from the earthquake was limited to a relatively small area near to Bam city,
within a 20-30 km radius. In the Bam area the majority of the buildings that suffered extensive damage were of
one or two storeys construction and were built from mud bricks or other masonry materials. This earthquake
highlighted the particular vulnerability of other cities in this earthquake region and in general the built
environment in Iran. The dramatic scale of the casualties associated with a relatively small affected region
underlines the fact that urgent measures need to be taken to safeguard the increasingly urbanised population
from the real risk posed by future earthquakes.
After the earthquake a field investigation carried out by the author from 11th to 17th of January 2004. The main
purpose of this paper is to record and comment upon the causes of the various types of damage observed in the
buildings and other structures and to determine what lessons can be learnt from this earthquake.
2. GEOLOGICAL AND SEISMOLOGICAL FEATURES
The Iranian plateau is part of the major Eurasian plate with the tectonic setting of the region dominated by the
collision of the Arabian, Eurasian and Indian plates. The Arabian plate is moving northward against the
Euroasian plate at a rate of approximately 30 mm/year with deformation of the Earth’s crust taking place across
a broad zone 1000 km wide, that spans the entire region of Iran and extends into Turkimanestan in the Northeast
of Iran [3]. Earthquakes occur as a result of both reverse and strike-slip faulting within the zone of deformation.
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
The Bam area is part of the Lut-e-Zangi Ahmad desert that has hot summers with temperatures up to 50 oC and
winters with below freezing temperatures. The geomorphology of the region also includes a range of mountains
to the North of Bam extending northwest and also the Jebal-e-Barez mountain range to the Southwest of Bam
extending in a Northwest-Southeast direction. Water sources within these mountain ranges are the main
suppliers for the Qanat system in Bam, Baravat and their satellite villages. The seasonal Posht-Rood river that
flows to the North of Bam city is dry during much of the year.
The geology of the region is dominated
by lithologies ranging from recent
Quaternary alluvium to Eocene
volcanic rocks [4]. Based on 1:100,000
Bam geological map sheet published
by Geological Survey of Iran (GSI) [8]
Bam and Baravat and surrounding
areas are covered by coarse brown
sandstone deposits. The northeast area
of the Arg-e Bam is founded on tuff
and traciandesite rocks while other
parts are built on alluvial deposits [9].
The thickness of alluvium in Bam
varies between 0 and 30m. The main
tectonic feature in the area is the Bam
fault that can be identified on the
geological map, between Bam and
Baravat, along the line separating older
fans from younger sediments.
54.00o
56.00o
58.00o
60.00o
32.00o
840608
5.3
980314
6.6
810728
7.1
30.00o
891120
5.7
Bam
810611
6.7
28.00o
Figure 1: Regional seismology of Bam city (Courtesy of IIEES [5],
In the central east area of Iran The dates and magnitude of the earthquakes have been
deformation from tectonic activities reproduced from references [6] and [7])
takes place along faults, which have
predominantly North-South and Northwest-Southeast directional trends. Although no major historical or recent
earthquakes have been reported near Bam or along the Bam Fault, northwest of Bam and within a range of
150km there have been several destructive earthquakes during past 30 years. The major faults in this region
include the Nayband fault with a North-South trend, the Kuh Banan fault which trends Northwest-Southeast and
the Gowk fault that starts at the junction of the two aforementioned faults and trends in a North-South direction
towards the Jebal Barez mountains in the Southwest of Bam, as shown in Figure 1[7]. Five major earthquakes in
recent years are shown in Figure 1 of which four are associated with the Gowk fault. There are also historical
earthquakes reported in the region: The Sirch-Hassan-abad 1877 earthquake (Ms 5.6), about 130km Northwest
of Bam, the Laleh Zar earthquake (1923, Ms 6.7) that killed 200 and the Golbaf earthquake (1949, Ms 6.0) [2].
3. RECORDED GROUND MOTIONS
Out of the 78 instruments within a 300km radius of Bam, the main shock of the Bam earthquake was recorded
by at least 24 instruments. The maximum uncorrected accelerations recorded at Bam station (58.35E, 29.09N)
were 0.82g, 1.01g and 0.65g in the longitudinal (East-West), vertical and transverse (North-South) directions,
respectively. The recording instrument was located on the ground floor of two-storey Governor’s office building
in Bam which has recorded a ground motion very close to the epicenter. Uncorrected ground accelerations are
shown in Figure 2, which show severe vertical and fault normal accelerations [11]. The duration of the strong
motion based on Evolution of Arias Intensity was about 8 seconds for the fault-normal component [10].
The earthquake epicenter was located at 29:00N-58.34E, about 185km Southeast of Kerman with a focal depth
of about 10km [11]. Studies by Talebian et al [13] based on interferograms derived from coseismic satellite
maps suggest that the fault responsible for the Bam earthquake was a blind strike-slip fault located about 5 Km
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
to the west of the visible surface traces of the Bam fault. Accordingly the causative fault extends beneath the
Bam City from the south.
Uncorrected PGA
A pre-shock was recorded in Bam station about 53 minutes
before the main shock with maximum horizontal acceleration of
0.017g and maximum vertical acceleration of 0.08g, and an
estimated depth of 10km. The pre-shock was sufficiently strong
that a small proportion of the population took precautionary
measures by staying out of their homes. More than 60
aftershocks were recorded in the 6 days following the main shock.
The largest aftershock happened about one hour after the main
earthquake with a magnitude of 5.3[11].
0.82g
1.01g
3. OVERALL DAMAGE PATTERN
Damage from the Bam earthquake was mainly concentrated in
Bam City and the surrounding villages. Strong motion
attenuation in the East-West direction was significantly higher
than in the north-south direction. Total affected population
estimated to be 145,500 [12]. The most of the population centers
other than Baravat and Bam’s satellite villages (less than 10km
from Bam city) fall beyond the high intensity zones.
0.65g
Figure 2: The accelerographs for the
main shock recorded in Bam station [9]
In overall terms the damage observed in the city of Bam varied
depending upon location and building construction. The damage was very severe in the old part of the city in the
Northeast and around Bam Citadel where the construction was dominated by adobe type building structures.
Severe damage was also observed in newly constructed parts of the City in the Southeast. The aforementioned
areas were densely populated while most of the Southern and Western parts had dual land use with many so
called ‘garden-houses’ with palm trees planted in large courtyards. The damage in these areas was
comparatively low, varying between 30% and 70%.
Ground failures and deformations were not significant in the Bam Earthquake. Lifeline infrastructure in the
earthquake stricken region and within the city of Bam performed reasonably well, with the exception of the
Qanats, a traditional irrigation system, which suffered severe damage. The damage to roads, bridges, railway
and airport was minor. Many streets and most of the alleys were blocked after the earthquake due to debris from
the damaged buildings. The airport was out of operation for a few hours after the earthquake due to damage to
the airport control tower but later played a major role in the rescue and relief operations. Power transmission,
electricity and telecommunication networks, and water distribution system also suffered minor damage.
Before the Earthquake there were more than 120 qanats (subterranean tunnels that tap groundwater and lead it to
human settlements and agricultural lands under gravity) in the Bam region. Of the 65 qanats supplying water to
Bam’s world-famous date gardens, 25 experienced some local collapse and subsequently dried up. The
remaining qanats suffered damage approaching 40 to 50%. The agro-economic effect on the Bam region was
therefore particularly severe due to dependency on agricultural products such as dates.
Despite the fact that the most of the area near to Bam has a high proportion of loose sand and silt deposits there
were no reports of damage due to liquefaction after the earthquake. This can be attributed to the low level of
ground water in most parts of the region. Some minor effects from landslides in the form of separated earth
blocks and falls in dry natural drainage channels were observed in southeast of Bam and near to Baravat.
4
DAMAGE TO ARG-E-BAM
Arg-e-Bam (Bam Citadel) is the oldest and largest mud-brick structure in the world and thought to be around
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
2000 years old. The complex is located in the Northeast of the modern city and historically was the old city of
Bam, partly inhabited until 180 years ago. The Arg was built out of clay, mud brick, straw and trunks of palm
trees at the foot of a huge rocky outcropping and is surrounded by a rampart, consisting of 38 towers, and deep
trenches that provided an effective defensive barrier against possible attack. The complex covers an area of
about 200,000m2 and incorporates three specific sections [15]. Since 1973 routine repairs were conducted on
Arg to conserve this magnificent ancient mud brick structure and before the earthquake on 26 December 2003 it
had become one of the most popular tourist destinations in the southeast of Iran.
The Arg suffered extensive damage during the earthquake. Approaching from the Southwest, some parts of the
external walls of the complex were reduced to rubble, barely resembling the shape of the original construction.
By climbing on top of their remains one could clearly see the complex’s debris filled courtyard. The fact that
this structure had been standing undamaged for around 2000 years has been taken by some [1, 22] as evidence
that the Bam region had not experienced any earthquake of a similar intensity to the Bam earthquake during this
period. Figure 3 shows damage to Arg-e Bam after the bam earthquake.
Before earthquake
After earthquake
Figure 3: Damage to the Arg-e-Bam after Bam Earthquake
5
PERFORMANCE OF BUILDING STRUCTURES
The Bam earthquake is by far the most devastating earthquake in the history of the region around Bam. Located
at a relatively short distance from the epicenter, Bam city experienced particularly intense local ground shaking.
Despite the enforcement of seismic code in the region, many of the building structures were too old for their
construction to be controlled by modern standards. However a significant proportion of the new buildings
should have been designed and constructed taking account of the seismic requirements in the Iranian codes.
It is well known that the building construction practice in Iran and more specifically in small cities like Bam has
been poorly regulated and monitored. There is clear evidence of this from the observations made after the Bam
Earthquake. The widespread total damage to the majority of newly constructed buildings in the private sector
contrasts strongly with the more limited damage to the few structures, mainly in the (well-regulated) public
sector, that remained standing. This strongly suggests that poor construction practice played a major role in the
vast destruction and high death toll within the city.
In the following a summary of observations made to the extent of damage to building structures is presented.
5.1 Seismic Regulations and Construction Practice
Following the Buein-Zahra Earthquake of 1st September 1963 which killed more than 12000 people, the first
Iranian regulations for seismic design appeared in the guideline “Seismic Safety Code for Building” published
by the then Ministry of Housing and Reclamation in 1967. The seismic load calculation procedures for this code
were subsequently published as “Standard No. 519, minimum loads for buildings” by the Planning and Budget
Organization giving mandatory minimum loading standards for new building structures. The first edition of the
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
current “Iranian Code for Seismic Resistant Design of Buildings, Standard No. 2800” [16] was published by the
Building and Housing Research Centre (BHRC) in early 1988 and since then has contained the official
requirements for seismic design in Iran. The second edition was published in 1999 while the third edition has
recently been published.
There are a number of factors affecting building construction in the Bam region such as cost, climate
requirements and the availability of suitable materials. Types of construction range from mud-brick and adobe
structures, fired brick masonry structures to reinforced concrete and steel structures. In terms of normal building
practice, private homes are built by their owners who hire the builder and unskilled labour. There are no
requirements for the registration of builders or contractors for these buildings and as a result there is no adequate
system to prosecute negligent builders. However there is legislation in place requiring the production of
construction drawings, for the control of construction by authorized professionals and for final approval of
drawings by local authorities prior to the start of the construction. Nevertheless, in real terms, there is little
control on the construction process itself.
5.2 Building Damage
The most dominant structural systems in the Bam region are:
• Adobe buildings: built from adobe materials and unfired mud bricks. Most with a vaulted roof system.
• Masonry buildings: built from fired bricks or concrete blockwork as the main load bearing system and
normally combined with a jack-arch roof system.
• Steel structures: Typical construction includes frame structures with steel beams and columns and
sometimes a braced framing system to resist the lateral loads.
• Reinforced concrete structures: Only a limited number of these structures exist in Bam mostly used for
public buildings or government offices.
Outside the city, in the villages, adobe and masonry buildings are the main structural types.
5.2.1. Adobe buildings
Adobe and mud bricks are one of the oldest and most widely used building materials in the southeast of Iran.
The adobe buildings normally have domed or vaulted roof system. For these roof systems the final finished
level may still be flat. In this structural system thick and stiff walls provide the main load bearing system. From
structural standpoint adobe structures are bulky and heavy. More importantly the roof can be very heavy due to
the complex vaulted system, or due to additional weight accumulating by the application of insulating layers
normally added every few years. Soil is a weak and brittle construction material and will disintegrate easily if
subjected to strong vibration. Under seismic loads the heavy walls and roofs develop large inertia forces that
cannot be resisted by walls often resulting in large cracks or collapse. Although the vaulted roofs perform well
in transferring gravity loads, due to utilization of mud bricks in compression, they are not well suited to
transferring horizontal seismic loads or strong vertical
seismic loads. The result is a sudden collapse of the
structure with insufficient time for evacuation and a
dusty atmosphere afterwards, further reducing the
survival prospects for the victims trapped under the
building. The majority of these buildings collapsed in
the old district of Bam, leaving a flattened area.
Figure 5 shows an example of damage to adobe
buildings in the Bam earthquake. Severe damage to
total collapse was seen in Bam city and Esfikan village
while in Nartij village most of the buildings were
standing with only moderate damage with a smaller
proportion suffering collapse. Although most of the
buildings with vaulted roofs collapsed within the city,
Figure 5: Total collapse of adobe buildings in Bam
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
few had performed well and remained standing after the earthquake with only moderate damage. Vaulted roofs
were also seen in fired brick masonry buildings, again in some cases with good performance. From those
vaulted roof mud brick buildings that did not collapse it could be seen that most of the vault infills and end walls,
perpendicular to the direction of vault axis, had collapsed. This suggests that the deformation mode of the
vaulted roofs and their supports differs from those of the perpendicular walls, and the lack of a shear transfer
mechanism between these two parts resulted in the separation and subsequent collapse of the walls.
5.2.2. Masonry buildings
Masonry buildings constructed of fired bricks have become one of the most common construction methods in
Iran since the early 1960’s. Masonry walls are the main load bearing structural elements, and more often than
not, are combined with a partial steel frame consisting of a few columns in plan with steel beams spanning
between the columns and walls. The most commonly used flooring system consists of steel joists (standard I
beams) at approximately 1m spacing and fired brick jack-arches between the joists. This flooring system is also
very common in steel structures. The other common floor system is one with the floor constructed from
prefabricated reinforced concrete joists, about 50cm apart, with hollow bricks between the joists and an in-situ
reinforced concrete slab topping. In central and south-eastern Iran fired brick vaulted roofs are also very
common practice for the construction of masonry structures.
Since the introduction of modern seismic design codes in
Iran, masonry buildings are now required to have
horizontal and vertical ties. The use of horizontal ties in
Iran started in the 1960s and the use of vertical ties
began in the 1970s. However this practice took much
longer to be implemented in rural regions such as Bam.
Many of the older masonry buildings in Bam have been
constructed without ties and even some buildings,
constructed as recently as six months before the
earthquake, were found to be without ties. For masonry
structures without ties observed main failure types
include: a) horizontal displacement of simply supported
joists in jack-arch system resulting in collapse of arches
between the joist (b) horizontal movement and slippage Figure 6: A residential masonry building without
of the whole roof system on the walls resulting in total ties in Baravat. Construction finished about 6
collapse of the building. In addition, poor material months before the earthquake
properties and workmanship were a common factor and a
contributory cause to the damage experienced by many structures. Mortar materials used in older masonry
buildings are lime-clay, lime-sand or lime-sand-cement. Although in more recent construction sand-cement is
predominant, lime-sand is still used in some private houses. An example of masonry building total collapse is
shown in Figure 6.
5.2.3. Steel structures
The Iranian seismic code only permits masonry structures up to two stories provided that they satisfy other
specific limiting criteria. Beyond this limit steel frame structures are normally the first choice for engineers and
owners. The single most important issue for these structures is the lack of welding quality control and generally
poor workmanship. The secondary consideration is the lack of proper design due to the lack of seismic training
for designers, in relation to the design of structural and also the non-structural elements. Many of the structures
incorporating bracing systems to resist the lateral loads performed well and survived without collapsing and in
some cases without any damage to their glassy facades. Conversely, other steel structures in the same
neighbourhood suffered total collapse or severe damage. Simple frame structures that were obviously badly
constructed were among the most severely damaged structures. In some recently built steel structures flaws in
the design and/or construction had caused excessive deformations resulting in extensive damage to the internal
and external walls or in the worst cases total collapse.
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
Figure 7 shows damage to Kimia Building which is a
combined residential-commercial building with braced
steel frames in Bam city centre. The building had 5 stories
above ground towards the front but only 4 stories in the
rest of the building. Two to three stories were flattened
during the earthquake. This building is a typical example
of the effect of geometrical, stiffness and mass
irregularities which resulted in large lateral and torsional
displacements. This combined with poor quality of
welding have contributed to total collapse of the structure.
5.2.4. Reinforced concrete structures
Figure 7: Kimia building in Bam
In Bam reinforced concrete structures were rare and of
these very few, if any, were evident of being used for residential purpose, with the majority used for government
or other communal buildings. Generally they performed well and survived the intense force of the earthquake
with only minor to moderate damage to non-structural members. However poor design, materials and
construction quality caused severe damage and partial collapse to newly constructed reinforced concret Imam
Sadegh mosque in Bam
5.3 Stairwell extensions
Failure of the rooftop extensions to staircase structures was
common in the Bam Earthquake. The largest damage to these
structures was seen in masonry buildings where the structure is
merely an extension to the staircase walls and covered with a flat
or inclined jack-arch roof spanning between the two side walls
that run parallel to stair axis in plan. The other walls normally
have large openings for a door to the roof top on one side and a
window on the other side, which make the structure very flexible
and weak in the direction parallel to these walls, unless a proper
framing system is implemented. The weakness in this direction
has resulted in detachment of the whole staircase extension from
the building below, large movements and also instances of Figure 8: An Example of the performance
of staircase extension structures in Bam
collapse. The use of vertical and horizontal ties does not
necessarily provide sufficient framing for the staircase extension to resist the earthquake force in the direction
perpendicular to the stair axis. Due to the importance of the staircase structures in providing safe evacuation
from buildings, and since failure in these structures was a common occurrence in the Bam Earthquake the issue
of proper design of these structures and the provision of clear design guidance needs to be considered by local
authorities.
6 LESSONS LEARNED AND CONCLUSIONS
Despite the history of tragic earthquakes it appears that the issue of seismic risk has not to date been addressed
particularly effectively. This failing runs through every level of society and it is reasonable to ask why the
obvious lessons of the earthquakes are being ignored. Perhaps the intermittent nature of devastating earthquakes
tends to create a culture of acceptance of the status quo and failure to take responsibility in a society which has
mainly focused on short term needs. It is obvious, that for any plan to work successfully this culture needs to
change and the requirements for fundamental earthquake hazard reduction need to enter national consciousness
at all levels of society and then be enforced with minimum tolerance of negligence.
The experience of Bam earthquake highlights once again similar causes and problems in the disaster
preparedness system and the deficiencies in current construction practice, similar to most high casualty recent
earthquakes. The following are specifically noted:
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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
• Traditional mud-bricks buildings: Most of these buildings completely collapsed during the earthquake. A
considerable number of buildings in central and eastern provinces, more specifically in villages, are of this type
and will not resist similar magnitude earthquakes. Therefore, it is essential that any earthquake hazard reduction
programme in the region take these into consideration.
• Non-engineered buildings built of masonry or a combination of masonry and steel without any specific seismic
considerations: A good majority of the non-traditional buildings in many parts of Iran are of this type and are
vulnerable to earthquakes. An active retrofit and renewal strategy should be formulated.
• Seismic design code: The implementation of the code is a major issue and in this regard effective training of
professionals, providing additional guidelines for the code and effective construction control are essential.
• Lack of skilled labour and construction professionals: Most of the people working in the construction industry
are unskilled and unlicensed. This results in poor material production and construction. A process should be
implemented to train and licence professional working in construction.
Last but not least, as a country with high seismic risk and with its own seismic characteristics it is also necessary
to invest in scientific institutions and research facilities in earthquake engineering to pave the way for technical
and professional people to tackle the country-specific problems in seismic hazard reduction.
7
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